1. Field of the Invention
The present invention relates to an LC filter, and more particularly, to an LC filter which utilizes the series resonance between a capacitor and an inductor to achieve a strong attenuation at particular frequencies, and to maintain the effects thereof at the high frequency range.
2. Description of the Related Art
Among LC filters, there is a laminated LC filter which includes a coil portion and a capacitor portion in a ceramic made by laminating ceramic sheets having electrode patterns forming a coil and a capacitor.
An example of such a laminated LC filter is disclosed in the Japanese Unexamined Patent Application Publications No. 10-13180 and No. 10-200357 which has an equivalent circuit as shown in FIG. 12. In this equivalent circuit, two coils L1 and L2 are connected in series between terminals 12a and 12b, and an equivalent coil L3 constituted by the mutual inductance M between the coils L1 and L2, a capacitor C, and a minute coil (minute inductor) L4 for adjusting the resonance frequency are connected in series between the node of the two coils L1 and L2 and a ground.
As shown in FIG. 12, if the mutual inductance M between the coils L1 and L2 is positive, this equivalently means that there exists a negative inductance (xe2x88x92M) (equivalent coil L3) between the node of the coils L1 and L2 and the ground. The resonance frequency, therefore, can be adjusted by connecting the minute coil (minute inductance) L4 in series with the capacitor C.
As shown in FIG. 13, if the mutual inductance M between the coils L1 and L2 is negative, this equivalently means that there exists a positive inductance (+M) between the node of the coils L1 and L2 and the ground, and the positive inductance generates a series resonance with the capacitor C. In this case, the resonance frequency can be adjusted by adjusting the coupling state of the positive inductance with the capacitor C and/or the coils L1 and L2.
However, in the LC filter as described above, at a specific resonance frequency, a strong attenuation is achieved by the resonance between the inductance and the capacitor C, but after the resonance point, the inductance connected in series to the capacitor increases the impedance, so that the effect provided by the capacitor becomes insufficient. Thus, the above-described LC filter cannot produce a sufficient attenuation effect over a wide frequency range while having a sufficient attenuation effect at particular frequencies.
Also, in the conventional laminated LC filter, the electrode patterns forming the coil portion and the capacitor portion are each designed, in consideration of the properties thereof, to include ceramic sheets on which these electrodes are printed and laminated in a desired order, and a magnetic flux passes through the electrode for the capacitor (capacitor electrode), that is, the capacitor electrode obstructs the magnetic flux. As a result, an eddy current occurs in the capacitor electrode, and the eddy current loss substantially reduces the inductance.
Among the conventional inductors suitable for large currents, there is an inductor formed by winding coated copper wiring around a ferrite ring core. To construct an LC filter using such an inductor, it is necessary to separately install a capacitor. This increases the number of components, and further a stray capacitance and a residual inductance are generated because the inductor and the capacitor are connected by connecting separate components, resulting in insufficient attenuation characteristics.
Accordingly, to overcome the above-described problems, preferred embodiments of the present invention provide an LC filter in which the stray capacitance and the residual inductance of a capacitor are minimized, and which achieves a strong attenuation property at particular frequencies by producing a series resonance by an equivalently generated mutual inductance, a residual inductance, and a capacitance, and maintaining the attenuation effect over a high frequency range by setting the value of the coupling coefficient between the two coils such that the coupling between the two coils cancels all residual inductance. The LC filter is suitable for high-current applications.
The laminated LC noise filter in accordance with a first preferred embodiment of the present invention includes a magnetic body and two coils disposed in the magnetic body, which are defined by a metallic plate having a spiral shape, and which are connected in series to generate a mutual inductance therebetween. A capacitor electrode plate, having a shape which is included within the shape range of the coils when viewed from the axial direction thereof such that it does not interfere with the magnetic flux generated by the coils, is disposed in the vicinity of the connection point of the two coils in the magnetic body such that at least a portion of the capacitor electrode plate is opposed to a portion of the coils.
By arranging the two spiral shaped coils to be serially connected to generate a mutual inductance, and disposing the capacitor electrode plate, in the vicinity of the connection point of the two coils so as to be opposed to a portion of the coils, the coils also act as a capacitor electrode. This reduces the number of components, and eliminates the need to provide a capacitor independently of the coils, resulting in greatly reduced residual inductance.
Also, because the capacitor electrode plate is configured to be included in the shape of the coils when viewed from the axial direction thereof, that is, a shape which does not to extend beyond the planar shape (projected shape) of the coils in plan view, the magnetic flux generated between the two coils does not pass through the capacitor electrode plate. This prevents a decrease in inductance due to eddy current loss, and achieves a greatly increased inductance.
Furthermore, by arranging the mutual inductance between the two coils to be positive or negative, an outstanding attenuation property at a high frequency range is achieved, and a strong attenuation at particular frequencies by a series resonance is also achieved.
In the LC filter of various preferred embodiments of the present invention, there is no particular limit to a specific shape or number of the capacitor electrode plates. The shape and number of the capacitor electrode plates is selected depending on the application.
Also, in the LC filter of various preferred embodiments of the present invention, there is no particular limit to the number of turns of the two coils. The number of turns of the two coils may be freely determined depending on application.
If the LC filter of various preferred embodiments of the present invention has an excessive coupling coefficient between the two coils, it will not achieve the effect of a T type filter which includes a distributed constant element, but it will possess only the property of a single coupled coil. Therefore, it is preferable to adjust the coupling coefficient between the two coils to an appropriate value. To enable the resonant frequency to be adjusted without sacrificing the attenuation property, it is preferable to set the coupling coefficient between the two coils in the range from approximately xe2x88x920.1 to +0.1.
An LC filter in accordance with a second preferred embodiment of the present invention includes a magnetic body and two coils which are formed of a metallic plate having a spiral shape, which are connected in series to generate a mutual inductance, and which are aligned in the magnetic body in the axial direction thereof such that the central axes of the two coils are substantially the same. A capacitor electrode plate, which is configured to be included within the shape of the coils when viewed from the axial direction thereof so that it does not interfere with the magnetic flux generated by the coils, is disposed between the two coils in the magnetic body so that at least a portion of the capacitor electrode plate is opposed to a portion of the coils.
By disposing the two coils aligned in the axial direction thereof such that the central axes of the two coils are substantially the same, and by inserting the capacitor electrode plate between the two coils, an LC filter is provided which exhibits an effect similar to the LC filter in accordance with the first preferred embodiment. This enables the present invention to be highly effective.
The LC filter in accordance with a third preferred embodiment of the present invention includes a magnetic body and two coils which defined by a metallic plate having a spiral shape, which are connected in series to generate a mutual inductance therebetween, and which are disposed in the magnetic body to be spaced from each other in a direction that is substantially perpendicular to the axial direction of the coils so as to be adjacent to each other when the coils are viewed from the axial direction thereof. A capacitor electrode plate, having a shape which is included within the shape of the coils when viewed from the axial direction thereof such that it does not obstruct the magnetic flux generated by the coils, is disposed in the vicinity of the connection point of the two coils in the magnetic body such that at least a portion of the capacitor electrode plate is opposed to a portion of the coils.
As described above, by disposing the two coils at the locations displaced from each other in the direction that is substantially perpendicular to the axial direction of coils so as to be adjacent to each other when the coils are viewed from the axial direction thereof, and by disposing a capacitor electrode plate having a shape which does not obstruct the magnetic flux generated by the coils in the vicinity of the connection point of the two coils, an LC filter is provided which exerts an effect similar to the LC filter in accordance with the first preferred embodiment. This enables the present invention to be highly effective.
Preferably, the LC filter of various preferred embodiments of the present invention includes a magnetic body constructed of a magnetic resin obtained by kneading a resin with magnetic powder.
By using a magnetic resin obtained by kneading a resin with magnetic powder as a magnetic body, an LC filter having two coils and a capacitor electrode plate embedded in a magnetic body (magnetic resin) by an injection molding method or other suitable method is provided, and thus a high-inductance LC filter is efficiently produced, which permits the present invention to be highly effective.
Other features, characteristics, elements and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the present invention with reference to the attached drawings.